The present invention relates generally to alignment targets, and more particularly to methods utilizing alignment targets in the fabrication of integrated circuits on semiconductor wafers.
The trend in fabrication of integrated circuits (ICs) has been toward increased circuit complexity and decreased specific circuit size on a larger wafer. Production processes for such ICs have often required a plurality of masks to be successively applied to a semiconductor wafer. Typically, sub-micron alignment tolerances are required between mask images. Alignment of the wafer with a predetermined reference location and/or the masks has thus become an important step, particularly in a conventional step-and-repeat projection printer. Obtaining rapid and accurate alignment between a single mask and wafer and among several masks and the wafer has been a continuing problem in IC lithography as smaller circuit sizes require smaller alignment tolerances.
Several automatic systems for obtaining high precision alignment have been proposed. Three primary means of precision optical alignment include: optical interference, for example, laser illumination of diffraction gratings; incident laser beam deflection at mask edges; and digital processing of video signals from a wafer target. The first method often involves complex and specifically dimensioned Fresnel zone plates disposed on the mask and/or the wafer. These zone plates reflect incident radiation, acting as a focusing lens to create a position indicating dot. Bright images must be reflected at every wafer level even when coated with photoresist in order for this method to work efficiently and quickly. Surface irregularities also present accuracy problems for this alignment method.
Digital processing of video signals has become a more practical, flexible, and cost effective means of wafer alignment in production of many high speed, large memory ICs. Further, this method can also now provide as much alignment precision as optical interference methods. A typical, conventional automatic wafer-to-reticle alignment system using digital processing of video signals is shown in FIG. 1. Such a system is presently commercially available in, for example, the Electromask 700 SLR/800 SLR Wafer Steppers, and only a brief description will follow herein.
Semiconductor wafer 10 is mounted beneath imaging lens system 20 and optically aligned along axis 25 with chip or die reticle 30. Projector 40 provides a light source which causes the image of reticle 30 to be reduced through lens system 20 and projected onto wafer 10 as an individual chip or die 12 at each required location.
Semiconductor wafer 10 is shown as having a plurality of individual chips or die 12 formed thereon. For convenience of description, only a few such die 12 are shown in FIG. 1, but it will be recognized by those skilled in the art that any number of die may be so formed, depending upon the size of wafer 10 and the reduction power of lens system 20. Alignment targets 15 are shown as associated with each die 12, although if alignment tolerances are large or if surface irregularities, such as wafer bow, are not a problem fewer alignment targets 15 than die 12 may be sufficient to assure global alignment. Targets 15 may be projected onto wafer 10 by any convenient, conventional means and may be positioned anywhere within associated die 12, including the scribe area, as shown in FIG. 1.
Wafer/reticle alignment is provided by a video camera 50 which views targets 15 through window 32 in reticle 30 and through lens system 20. The video signal output from video camera 50 is digitalized by high speed A/D converter 55, preprocessed in hardware unit 60, and transferred to CPU 65 to calculate positional information and issue any necessary correction signals for wafer/reticle alignment to micropositioners 70 and 72. These micropositioners may include, for example, laser interferometers. Although two micropositioners 70 and 72 are shown in FIG. 1, in some applications only a single device may be employed. Further, CPU 65, micropositioners 70 and 72, and hardware unit 60 may also be readily employed to step-and-repeat reticle images to entirely different wafer positions, rather than merely align once at a particular position.
Such prior alignment systems typically employ target patterns having vertical and horizontal lines 17 and 18, respectively, each with significant dimensions, such as line width. It is necessary to scan the absolute magnitude both of these dimensions to provide sets of data indicating vertical and horizontal positions within the target pattern. An example of such a target 15 viewed through reticle window 32 is shown in FIG. 2. Line by line intensity scans are made of the video signal output of video camera 50 and added together to give digital profiles of the absolute magnitudes of the pattern dimensions on both horizontal and vertical axis of the video signal. These digital profiles may be compared with a reference profile to determine the position of the center of reticle window 32 relative to a reference point on wafer 10, the target center, for example, and the necessary corrective signals to be issued for micropositioners 70 and/or 72 to reposition wafer 10 and/or reticle 30 to achieve alignment with that reference point. However, if wafer 10 and reticle 30 are sufficiently misaligned, the portion of target 15 seen by video camera 50 through reticle window 32 will contain either line 17 or line 18 or no line at all. Without the intersection of such horizontal and vertical lines, CPU 65 cannot determine the direction and distance of the interrogation position from a reference point, and thus cannot provide alignment correction signals to micropositioners 70 and 72.
Also while such an automatic alignment system does provide an increase in alignment speed and accuracy over prior systems, as the IC fabrication technology has progressed, the need has arisen for even faster and more accurate alignment means. Furthermore, it is desirable to provide such a system that is compatable with a wide variety of pre-existing hardware and which can detect a range of errors largely defined by the user for specific production runs.